OBJECTIVE The goal of this study was to determine whether high E2 levels after controlled ovarian hyperstimulation affect TSH. METHODS Patients completing ART cycles between April-October 2010 were eligible for this cohort study. 180 patients were recruited however those with known thyroid disease were excluded. The final analysis included 154 subjects. Blood was collected at each visit during the ART cycle as well as at the pregnancy test. Samples were frozen at -20 °C and analyzed together for E2 and TSH using the same assay kit once all patients had completed their cycles. All participants were treated at the McGill University Health Center. A paired t-test was used to study the difference in TSH levels recorded at maximal and minimal Estradiol levels during ovarian stimulation. Multiple regression analysis was then used to determine if factors such as anti-thyroid antibodies and ovarian reserve measures affect this change in TSH. We used multiple imputation methods to account for missing data. RESULTS As E2 levels rose from low to supra-physiologic levels during treatment, TSH levels also rose significantly. This increase was clinically significant by the time of pregnancy test. The factors that potentially affected the change in TSH were: male factor/tubal factor infertility, type of protocol used as well as the presence of thyroid antibodies. CONCLUSIONS Although TSH increases during ART, this change only becomes clinically significant on the day of pregnancy test. Future studies should examine TSH changes specifically in certain "at-risk" sub-groups such as those with antibodies and known thyroid disease.